As well-known in the art, a double-acting jar of the type described comprises a first tubular member or outer housing adapted to be connected to the object and a second tubular member or inner mandrel adapted to be connected as part of a pipe string and arranged telescopically of the first member or housing and having a hammer arranged to deliver an upward blow to an anvil on the housing in response to raising and a downward blow in response to lowering of the mandrel with respect to the housing. In an hydraulic jar, the mandrel carries a detent mechanism arranged to slide within a restricted bore of a chamber in the housing in which hydraulic fluid is contained. Thus, the detent mechanism is of such construction as to permit only limited flow past it, during its travel through the restricted bore, so as to build up tension in the mandrel as it is raised therethrough or compression therein as it is lowered therethrough. Consequently, as the detent moves out of the restricted bore, the mandrel and thus the hammer thereon is released to deliver either an upward or downward blow to the anvil of the housing and thus the stuck object to which the housing is connected.
As also well-known in the art, the object stuck in the well bore may be a "fish" onto which the lower end of the housing of the jar is lowered for connection thereto. Or, the object may be a lower part of the pipe string itself and thus already connected to the housing of the jar.
It is well-known in the art to enhance the impact of such jars by means of a so-called jar enhancer comprising a second tubular member or inner mandrel having an upper end adapted to be connected to the well pipe string above it, and a first tubular member or outer housing arranged telescopically of the mandrel member to form an annular space between them and having a lower end adapted to be connected to the mandrel member of the jar (or to drill collars above the jar) for raising and lowering therewith. More particularly, the space may be a pressure chamber which is filled with a compressible fluid which may be a gas, such as nitrogen, or a compressible liquid, such as silicone, and a piston on the mandrel may sealably slide with respect to the bore of the housing to compress the fluid in the space above it as the mandrel is raised, in the case of an upward jar, or below it as the second member is lowered, in the case of a downward jar.
In a jar, the second tubular member or mandrel on which the hammer is carried has a certain amount of "free" travel following movement of the detent out of the restricted bore and as the hammer moves to strike the anvil. In jar enhancers prior to the invention disclosed in U.S. Pat. No. 4,846,273, the second tubular member or mandrel on which the piston is carried conventionally had a certain amount of "free" travel before it is moved into the restricted bore to accumulate energy.
In accordance with U.S. Pat. No. 4,846,273, however, it was proposed to provide a jar enhancer having an initial "free" travel at least as great as that of the jar, thereby insuring that accumulated energy was applied to the stuck object. Although representing a substantial advance over the prior art in this respect, the jar enhancer of the aforementioned patent is of relatively complex construction and expensive to manufacture, including a large number of parts which cause it to be quite long and difficult to service and repair. Also, at least in the version of the jar enhancer of FIGS. 6 and 7 of U.S. Pat. No. 4,846,273, the piston moving in the restricted bore is of necessarily complex construction to enable it to move in the opposite direction through the restriction following the jarring event.
Pending patent application Ser. No. 08/145,481, filed Oct. 19, 1993, and assigned to the assignee of the present application, discloses a double-acting jar enhancer of this general type which overcomes these and other problems with those of the prior art in that it is of simpler and less expensive construction. Thus, in accordance with the embodiment of the improved jar enhancer illustrated in the prior application, upper and lower longitudinally spaced seal rings are disposed within the annular space between the tubular members for sealably sliding with respect to equal diameter portions of each, the first member or housing adapted to be connected to the second tubular member or mandrel of the jar has an upper shoulder engagable with the upper ring to limit its movement in a downward direction and a lower shoulder engagable with the lower ring to limit its movement in an upward direction, and the second member or mandrel adapted to be connected to the well string above it has an upper shoulder engagable with the upper ring to raise the upper ring with it, upon raising of the second tubular member, and a lower shoulder engagable with the lower ring to lower the lower ring with it, upon lowering of the second tubular member. More particularly, means are provided to form upper and lower sliding seals between the first and second tubular members along the diameter portion of the first member to form an upper pressure chamber within the space above the upper seal ring and a lower pressure chamber within the space below the lower seal ring, whereby, with the upper and lower chambers filled with a compressible fluid, raising of the second member will compress the fluid in the upper chamber to enhance an upward jar and lowering of the second member will compress the fluid in the lower chamber to enhance the impact of a downward jar. Preferably, the shoulders on the first member are essentially adjacent the upper and lower seal rings when said second tubular member is in a neutral position.
Although the jar enhancer of the prior application represents a substantial advance over prior jar enhancers of this general type, there is a need in the industry, particularly as wells are drilled to greater depths, to be able to apply greater loads to the jar enhancer without exceeding its burst strength. Of course, one obvious solution would be to use multiple, end-to-end jar enhancers, but this would multiply the costs proportionately. On the other hand, due to the rigid constraints of space within the well bore, it is not practical to merely increase its diameter and thus the effective pressure-responsive areas in its fluid chambers. Consequently, it is the object of this invention to provide an improved jar enhancer of this type which is of such construction as to reduce the pressure in each fluid chamber by fifty percent or more for any given load without increasing its outer diameter or substantially increasing its cost.